Aluminum alloys in the 7000 series containing high amounts of zinc, copper and magnesium are known for their high strength-to-weight ratios and, therefore, find application in the aircraft industry. Such aPplications, however, result in exposure to a wide variety of climatic conditions necessitating careful control of working and aging conditions to provide adequate strength and resistance to corrosion, including both stress corrosion and exfoliation.
Conventional alloys of this type such as 7075, which contain 3 to 8 wt. % Zn, 1.5 to 5 wt. % Mg and 0.75 to 2 5 wt. % Cu, can be aged at a temperature of about 215.degree. F. to 250.degree. F. to obtain excellent strength properties in what is known as the T6 temper. High resistance to exfoliation and stress corrosion with some sacrifice in strength can be achieved by subsequently aging the alloy at temperatures of 300.degree. F. to 350.degree. F. for a sufficient period of time to achieve what is known as a T7 temper
In some instances, modifications of the above tempers have been successfully made by careful control of the amounts of alloying materials combined with the use of elevated temperatures. For example, Staley et al, U.S. Pat. No. 3,881,966, disclose and claim a method for obtaining a 7000 series alloy exhibiting high strength and high resistance to stress corrosion utilizing an aging temperature of 300.degree. F. to 380.degree. F. in combination with careful control of the ratio of alloying constituents.
Aging at lower temperatures to achieve the same results has also been accomplished using other alloying constituents. For example, Markworth et al, U.S. Pat. No. 3,794,531, teaches the use of single stage aging at a temperature of 212.degree. F. to 284.degree. F. by substituting 2% of the zinc content with cadmium.
The use of nickel as an additive in 7000 series alloys to achieve resistance to stress corrosion is taught by Zeigler et al in U.S. Pat. No. 2,403,037. This alloy is said to be aged in a single step at temperatures of 275.degree. F. to 350.degree. F.
While the use of a single step aging process has advantages from the standpoint of efficiency and energy conservation, the continuing search for alloys with higher strength properties compounds the problem of maintaining the desired resistance to stress corrosion as well as resistance to exfoliation.
Thus, the prior art seems to indicate that such properties can be maintained using aging temperatures of over 300.degree. F. with or without a prior aging step at lower temperatures. It was, therefore, quite surprising and disconcerting to find that the use of such aging (i.e., over 300.degree. F.) on high strength alloy similar to that disclosed in the aforementioned Staley et al patent (U.S. Pat. No. 3,881,966) resulted in lowered resistance to exfoliation if aging at temperatures of over 300.degree. F. was carried out over a short period of time (less than 7 hours at 325.degree. F., for example) and a loss in tensile strength if the aging period was extended.